A standard telescoping rock drill comprises a base, a guideway displaceable longitudinally forwardly and rearwardly on the base, a guideway actuator braced between the guideway and the base and energizable for longitudinal forward displacement of the guideway on the base between front and rear positions, a drill carriage displaceable longitudinally on the guideway between a front position and a rear position, and a drill actuator braced between the drill carriage and the guideway for longitudinal forward displacement of the carriage on the guideway. As described in C. Boom U.S. Pat. No. 3,880,244 and K. Fujukawa U.S. Pat. No. 3,356,871 various limit switches are provided to back out the drill once it reaches a full-forward position, that is with both the drill carriage and the guideway in full-forward positions on the guideway and base, respectively.
In such an arrangement the force exerted longitudinally forward by the drill on the face is a function of several factors, namely the force exerted by the drill actuator, the force of the guideway actuator, the friction between the drill carriage and the guideway, the friction between the guideway and the base, the masses of the drill and of the guideway, and the angle the drill is set at relative to the horizontal. The drilling force effective on the drill bit when only the drill actuator is being used to advance it is a function of the force exerted by the drill actuator, less the force lost to friction between the drill carriage and the guideway, adjusted for the angle the drill forms to the horizontal. The telescoping force when only the guideway actuator is being used is a function of the actuator force less the losses to friction between the guideway and the base adjusted for the angle the drill forms to the horizontal which in turn is a function of guideway and drill mass.
The result is that the force the drill bit exerts on the face can vary within a wide range, even when the various actuators are pressurized always at the same levels. When drilling uphill, that is with the rear end of the drill bit below the front end, there is a substantial loss in drilling efficiency and an increased likelihood of damaging the drill bit, as well as the possibility of hammering on emptiness, which can also break the bit tip. When drilling downhill the drilling force is uncontrolledly augmented so that the bit can overheat and break, and the hole can move readily off line. In general it is difficult to calculate at any time just what the drilling force will be because it is a function of so many variables and the automatic switching between operation of the guideway and drill actuators further complicates the equation.